There has been considerable interest recently in solid state lasers operating in the three micron region. This is primarily because the strong water absorption at 2.9 microns makes them useful for medical applications. Millijoule energies and microsecond pulse lengths at repetition rates of hundreds of hertz are typically desired Rare earth lasers based on either erbium or holmium can operate in this wavelength region.
Erbium lasers are well characterized, and have been pumped with flashlamps or with laser sources at several wavelengths. G. J. Kintz, R. Allen, L. Esterowitz, Appl. Phys Lett. 50, 1553-1556, (1987) and S. A. Pollack, D. B. Chang, N. L. Moise, "Upconversion-pumped Infrared Erinbum Laser", J. Appl. Phys 60(12), 4077-4086, 1986. The fact that the latter wavelength directly pumps the lower laser level is indicative of the complexity of these systems. Moreover, these lasers are relatively difficult to control because the population inversion is generated through a series of up- and down- conversion processes. Although efficient long-pulse or CW operation is possible, Q-switching tends to be inefficient. Up-conversion out of the upper laser level clamps the inversion near threshold and interferes with efficient energy storage. These lasers are also characterized by high thresholds, low repetition rates and a tendency to lase at more than one wavelength.
Holmium lasers are less well characterized, primarily because they do not lend themselves to flashlamp pumping. The Ho:YAG and Er:YAG systems are superficially quite similar. For example, the lifetimes of the upper and lower laser levels are quite similar in the two systems. The large differences in flashlamp-pumped performance in the two systems can be attributed to differences in the cross relaxation processes which occur in each material. However, the holmium ion has the interesting property that the upper state lifetime is shorter, and there are fewer favorable up-conversion processes occurring. Moreover, holmium has the interesting property that the laser upper level absorbs near 1.06 microns.
Lasers which can be pumped with neodymium lasers are of particular interest in cases where laser diode pumping is being considered. It is relatively easy to diode pump a well-behaved material, such as Nd:YAG, and to use the resulting beam for pumping. Laser diode photons have been converted to 1064 nm photons with efficiencies approaching 70%. At present, large diode-pumped lasers are quite expensive and therefore impractical for many applications. Today, small flashlamp-pumped neodymium lasers are readily available and can be used as alternative pump sources until laser diode-pumped devices become more cost effective
Several rare-earth lasers have been pumped with neodymium lasers. Ytterbium-sensitized materials have been pumped with 1.06 .mu.m lasers (i.e., Galant (1978) and Antipenko (1984)). Work with holmium and erbium has also appeared recently (i.e., Rabinovich (1989) and V. I. Zhenkov et. al., "Lasing in Y.sub.3 Al.sub.5 O.sub.12 :Er.sup.3+ (.lambda.=2.94 .mu.m) Crystals as a result of Selective Excitation of the Lower Active Level," Kvantovaya Elektron. (Moscow) 16, 1138-1140 (June 1989). In all these cases, the absorptions were quite weak. This is because it is difficult to find coincidences between the narrow absorption and emission lines.
Previous efforts to pump holmium lasers with neodymium lasers used intracavity pumping so that the weak vibronic holmium absorption near 1.08 .mu.m could be used for pumping. W. S. Rabinovich, S. R. Bowman and B. J. Feldman, "Laser Pumped 3 .mu. Ho:YALO Laser," Optical Society of America Annual Meeting, 1989 Technical Digest Series, Vol. 18, Washington, D.C., 1989), paper TU04.
Heretofore it has not been possible to flashlamp pump Ho:YAG at high holmium concentrations. When the holmium concentration was increased above about 10%, lasing ceased. A. A. Kaminskii, Laser Crystals, Springer-Verlag, Berlin, 1981, Chapter 7. In particular 2.94 micron lasing has not been observed in high holmium concentrations in Ho:YAG.
Co-doping experiments have been limited to holmium concentrations below 15%. M. Kh. Ashurov, Yu. K. Voronko, E. V. Zharikov, A. A. Kaminskii, V. V. Osiko, A. A, Sobol, M. I. Timoshechkin, V. A. Fedorov, and A. A. Shabaltai, "Structural, Spectroscopy and Stimulated Radiation of Yttrium-holmium-aluminum Garnet Crystals", Inorg. Mater., (USSR), 15, 979-983, 1979. In Ho:Ba Y.sub.2 F.sub.8, 2.9 micron lasing was not observed with 10% Ho. L. F. Johnson and H. J. Guggenheim, "Electronic- and Phonon-Terminated Emission from Ho.sup.3+ in BaY.sub.2 F.sub.8 ", IEEE J. Quantum Electron., QE10(4), 442-449, 1974. Lasing has been observed by Antipenko in Ba Yb.sub.2 F.sub.8 with 15% holmium, but it relied on a series of transfers from the ytterbium sensitizer. B. M. Antipenko, "Spectroscopic Schemes for Excitation of Laser Transitions having a High Quantum Efficiency of the Pumping Bands", Bull. Acad. Sci. USSR, Phys. Ser. 48, 30 124-129, 1984. Lasing in other crystals has been observed at low holmium concentrations A. A. Kaminskii, Laser Crystals, Springer-Verlag, Berlin, 1981, Chapters 4 and 7. In a 15% Ho, 1% Nd:YAG system, the .sup.5 I.sub.6 lifetime was 40 .mu.sec and the .sup.5 I.sub.7 lifetime was 170 .mu.sec. M. Bass, et. al. Appl. Phys. Lett. "Simultaneous Multiple Wavelength Lasing of (Ho, Nd):Y.sub.3 Al.sub.5 O.sub.12 ", 51(17) 26 Oct. 1987, 1313-15. Increasing the holmium concentration in the system any further, resulted in a dramatic loss of holmium lifetime which was incompatible with flashlamp-pumped operation.
Since high holmium concentrations enhance the efficiency of energy transfer processes, it might be expected that selective quenching can be efficiently carried out in highly concentrated crystals. It has been demonstrated by Johnson et. al. and by others that co-doping holmium with praseodymium, europium or neodymium can have the desired effect. This is because there is a resonant transfer which allows efficient transfer out of the holmium .sup.5 I.sub.7 level but no resonance for .sup.5 I.sub.6 deactivation. For example in praseodymium, the process: EQU Ho(.sup.5 I.sub.7)+Pr(.sup.3 H.sub.4).fwdarw.Ho(.sup.5 I.sub.8)+Pr(.sup.3 F.sub.2)
is nearly resonant. Since it involves two high-cross-section transitions, it can occur very efficiently.
However, the usual technique has been to use a low holmium concentration and then increase the praseodymium concentration until quenching occurred. This technique causes several problems. First, it is usually difficult to incorporate praseodymium into laser crystals. The ionic radius is significantly larger than holmium or yttrium, and the distribution coefficients tend to be low. If higher concentrations are introduced, they tend to result in distorted crystals of poor optical quality. In YAG, praseodymium concentrations are usually limited to a fraction of a percent Similar comments apply to neodymium as well. In the case of europium, there is a significant absorption near 3 microns; thus, even if it were possible to grow the crystal with a high quencher concentration, the resulting absorption might hurt the laser's performance.
Thus, previous attempts to flashlamp pump a holmium laser in the 3 micron region at high holmium concentrations have not been successful. Moreover, despite the fact that the mechanism of quenching in rare earth crystal is well understood, it has proven to be difficult to implement this theory in a practical laser. In addition, CW operation of a holmium laser in the 3 micron region has not been possible.